This invention relates generally to the field of fluid-solid contacting. More specifically, this invention deals with the delivery of fluids to beds of particulate material.
Fluid-solid contacting devices have a wide variety of applications. Such devices find common application in processes for hydrocarbon conversion and adsorption columns for separation of fluid components. When the fluid-solid contacting device is an adsorption column, the particulate material will comprise an adsorbent through which the fluid passes. In the case of hydrocarbon conversion, the fluid-solid contacting apparatus is typically a reactor containing catalyst. Typical hydrocarbon conversion reactions that may be carried out are hydrogenation, hydrotreating, hydrocracking, and hydrodealkylation.
Fluid-solid contacting devices to which this invention apply are arranged as an elongated cylinder usually having a vertical orientation through which an essentially vertical flow of fluid is maintained. Particulate material contained in this vessel is arranged in one or more beds. Fluid enters the vessel through an inlet located at an upstream end of the vessel. It is also commonly known to add or withdraw fluid from between the particulate beds. This is commonly done in adsorption schemes where the composition of the fluid passing between particle beds is changing or in hydrocarbon conversion processes where a quench system is used to cool fluid as it passes between beds.
Changes in the composition or properties of the fluid passing through the particular zone present little problem provided these changes occur uniformly. In adsorption systems these changes are the result of retention or displacement of fluids within the adsorbent. For reaction systems changes in temperature as well as composition of the fluid are caused by the particulate catalyst material contained in the beds.
Nonuniform flow of fluid through these beds can be caused by poor initial mixing of the fluid entering the bed or variations in flow resistance across the particulate bed. Variations in the flow resistance across the bed can vary contact time of the fluid within the particles thereby resulting in uneven reactions or adsorption of the fluid stream passing through the bed. In extreme instances, this is referred to as channeling wherein fluid over a limited portion of the bed is allowed to move in a narrow open area with virtually no resistance to flow. When channeling occurs, a portion of the fluid passing through the bed will have minimal contact with the particulate matter of the bed. If the process is one of adsorption, the fluid passing through the channel area will not be absorbed, thereby altering the composition of this fluid with respect to fluid passing through other portions of the absorbent bed. For a catalytic reaction, the reduction in catalyst contact time will also alter the product composition of fluid as it leaves different portions of the catalyst bed.
In addition to problems of fluid composition, irregularities in the particulate bed can also affect the density and temperature of the fluid passing through the bed. For many separation processes retained and displaced components of the fluid have different densities which tend to disrupt the flow profile through the bed. Nonuniform contacting with the adsorbent particles will exacerbate the problem by introducing more variation in the density of the fluid across the bed thereby further disrupting the flow profile of the fluid as it passes through the particle bed.
In reaction zones, temperature variations are most often associated with nonuniform catalyst contact due to the endothermic or exothermic nature of such systems. Nonuniform contact with the catalyst will adversely affect the reaction taking place by overheating or overcooling the reactants. This problem is most severe in exothermic reactions where the higher temperature can cause further reaction of feedstock or other fluid components into undesirable products or can introduce local hot spots that will cause damage to the catalyst and/or mechanical components.
Fluid flow into the vessel can disrupt the top surface of the bed. The disruption results from transverse fluid flow across the surface of the bed at velocities sufficient to move the individual bed particles. For a confined bed, this disruption or movement of the particles will cause the particles to abrade against each other generating smaller particles which are referred to as fines. These fines may increase pressure drop within the bed or escape from the bed thereby reducing the overall quantity of particles in the bed and possibly interfering with downstream operations. In unconfined beds, transverse fluid flow may also shift large quantities of particles so that the bed surface is highly irregular.
These transverse currents are the result of charging fluid into a relatively large diameter vessel through a relatively small diameter nozzle. Charging fluid into the vessel through a small diameter nozzle produces a high velocity jet that extends from the nozzle into the vessel. Impact of this jet on or adjacent to the surface of a relatively closed catalyst bed flares the fluid outward thereby producing eddy currents and fluid velocities transverse to the bed surface. The inlet effects associated with the relatively small diameter nozzle are compounded by the usual presence of an elbow just upstream of the nozzle which introduces another transverse velocity component into the fluid flow entering the vessel. The overall result of these inlet effects is often the piling up of particles around the periphery of the particle bed or the shifting of particles from one side of the bed to the other.
These detrimental inlet effects are avoided by uniformly dispersing the fluid as it enters the vessel. Uniform dispersal can be obtained by providing a sufficient length between the nozzle and the catalyst bed surface such that the fluid jet and any transverse velocities are substantially dissipated upstream of the particle bed. However, in most cases, it is impractical to provide the length necessary for dissipation of the inlet effects due to the excessive vessel tangent length that would be required. In fact, the trend in many industries is to decrease the length between the inlet nozzle and the particle bed surface in order to increase the total volume of particles in the vessel and thereby obtain greater fluid throughput or greater particle bed service life.
For these reasons, inlet distributors are commonly used to break up the fluid jet and redistribute fluid flow over the top surface of a particle bed. One such device is shown in U.S. Pat. No. 2,925,331 issued to Kazmierczak et al. where a fluid stream is downwardly directed onto the upper surface of the catalyst bed and passes first through a distributor consisting of a series of annular plates having inner diameters that progressively decrease in the direction of fluid flow so that portions of the fluid stream are in effect peeled off and redirected radially outward over the surface of the particle bed. It is also known in the hydrocarbon processing industry to attach cylindrical rings extending in the direction of fluid flow to the inner edge of the annular plates. Another type of distributor used to redirect and remix fluid flow upstream of a particle bed is shown in U.S. Pat. No. 3,598,541 issued to Hennemuth et al. and U.S. Pat. No. 3,598,542 issued to Carson et al. The Hennemuth distributor uses a series of circumferentially spaced holes to redistribute fluid within a fluid mixing device that communicates with the upper surface of a particle bed. The distributor disclosed in Carson uses a series of circumferentially spaced holes to radially discharge fluid across the upper surface of a particle bed. Thus, the prior art is well acquainted with a number of distribution devices for use in fluid solid contacting vessels.
Despite the use of different inlet distributors, bed disruption remains a problem. Distributors that use the annular plates or baffles of the Kazmierczak device reduce the severity of bed disturbances but have not eliminated it. Therefore, large scale shifting of particle bed surfaces, especially where fluid inlet velocities are high, still occurs. Such disruption is still known to occur even in cases where straightening vanes and other flow distribution devices are added to the upstream elbow as a means of eliminating a resulting transverse flow component. It has now been discovered that despite the presence of the baffles and additional redistribution devices, such as straightening vanes, fluid flow entering the vessel still remembers the change of direction that took place upstream of the inlet nozzle.